Dioctahedral silicates

Denitrification, 340,473 Desorption, 221,362 Diffuse double layer, 141 Model, 142-146 Thickness, 145 Variable-charge surfaces, 146 Constant-charge surfaces, 143-146 Diffusion, 298, 398 Film diffusion, 398 Particle diffusion, 398 Solution diffusion, 398 Dioctahedral silicates, 122 Dispersion, 367... 

The basic 2 1 dioctahedral silicate is pyrophyllite with formula Al2Si40io(OH)2. In muscovite, KAl2(Si3Al)Oio(OH)2, interlayer K compensates electrically for the A1 replacement for Si. lllites have a more variable composition with a general formula (H30,K) Al2(Si4 cADOio(OH)2. Nontronite is the iron-rich dioctahedral silicate with general formula M+Fe +(Si4 ji Alji )Oio(OH)2. 

Kaolinite, ideally Al2Si205(OH)4, consists of 1 1 layers, alternating sequences of silicate and hydrated Al-octahedra (dioctahedral) sheets. There is potential for disorder in the specificity of the site occupied by Al and in the stacking of the sheets and layers, which give rise to the polymorphs dickite and halloysite. 

The stability of dioctahedral montmorillonites is, of course, not uniquely a function of P-T conditions acting upon a given silicate mineral assemblage. Studies in the system Na H A O -Sit - O-Cl (Hemley, jet al.. 1961) shows that a high activity of sodium ion at given silica and hydrogen activities can destabilize beidellite. Hess (1966) extrapolates this hydrothermal study to atmospheric conditions where the range of H+, Na+ and SiC activities can determine the presence of an expandable phase. 

The second facies is marked by the instability of the fully expanding dioctahedral phases and the existence of a kaolinite-illite tie-line (Figure 48b). In this facies the siliceous alkali zeolites (other than analcite) become unstable, the compositional range of the trioctahedral expanding phases is reduced and aluminous 14 8 chlorite-"allevardite"... 

The 1 1 clay-mineral type consists of one tetrahedral sheet and one octahedral sheet. These two sheets are approximately 7 A thick. This two-sheet type is divided into kaolinite (dioctahedral) and serpentine (trioctahedral) groups. The kaolinite minerals are all pure hydrous aluminum silicates. The different members are characterized by the manner of stacking of the basic 7 A layers (Brindley, 1961b). 

Lazarenko (1940) described a hydrothermal aluminum-silicate mineral from the Donetz Basin which he called donbassite. This material is a variety of dioctahedral chlorite and the Nomenclature Committee (Bailey et al., 1971) considers that it has priority. 

The two layer silicates are divided into the kaolinite (dioctahedral) and serpentine (trioctahedral) subgroups. The dioctahedral minerals are hydrous aluminum silicates containing minor amounts of other constituents. The trioctahedral minerals vary widely in composition and isomorphous substitution is common however, these minerals are relatively rare and chemical data are limited. 

In the dioctahedral 2 1 sheet-structure silicate with the occupied sites more than 85% occupied by Al, the structure seems to be able to compensate for the internal strain and can grow to a considerable size. The Al octahedral occupancy values of muscovite (>1.7) and the 2 1 dioctahedral clays (1.3—1.7) indicate that there is little overlap. It is likely that the decreased amount of tetrahedral twist induced by increasing the size of the octahedral cations and octahedral charge (decreasing Al) determines that a clay-size rather than a larger mineral will form. The R3+ occupancy value can be less than 1.3 when the larger Fe3+ is substituted for Al. When Al occupancy values are less than 1.3 (65%), in the absence of appreciable iron, the internal strain is such that growth is in only one direction. The width of the layer is restricted to five octahedral sites. Sufficient layer strain accumulates within this five-site interval such that the silica tetrahedral sheet is forced to invert to accommodate the strain. 

IHite/Smectite. Another common intergrowth of sheet silicates is the mixed-layering of illite and smectite. As discussed above, illite and smectite are clay minerals whose basic structures resemble the mica muscovite. Their compositions may differ significantly from muscovite, but they generally have a lower occupancy of the interlayer sites than mica. Numerous other compositional differences are possible for smectite however, this discussion will be restricted to a dioctahedral illite and a dioctahedral smectite containing potassium and vacancies in the interlayer sites as given above. 

Many of the layer silicate clays common in soils are based on the mica structure (shown in Figure 2.9b) in which two tetrahedral sheets sandwich a single sheet of octahedrally coordinated cations. Consequently, they are termed 2 1 layer sihcates. Conceptually, it is useful to start with the neutral framework of the talc and pyro-phyllite structures, representing the trioctahedral (Mg in the octahedral sheet) and dioctahedral (AF in the octahedral sheet) members of the 2 1 group. These have the ideal formulae given below ... 

In general, weathering conditions tend to favor the gradual conversion of trioctahedral layer silicate clays (with dominantly Mg " and Fe " in octahedral sites) to dioctahedral layer silicates (with dominantly AP and Fe in octahedral sites). Layer charge reduction accompanies this process. Expected weathering sequences of the two common micas, biotite and muscovite, in leaching environments are ... 

The conditions necessary for artificial synthesis of smectites suggest the kind of natural environment in which these minerals will be found. This environment is alkaline as a result of restricted drainage and/or evaporative salt accumulation (see Chapter 8), so that the normally mobile alkaline and alkaline earth ions (Na, K, Ca, Mg) accumulate, along with silica. Neoformation of smectites from Na-rich saline water, and possibly illites from more K-rich water, is favored under these conditions. Other very silica-rich silicate minerals, such as attapulgites, sepiolites, and zeolites, are also known to fomi under these conditions. Less alkaline conditions may be necessary for Fe and Al-rich (dioctahedral) smectite formation, and a reducing environment may assist crystallization. 

Moderately Alkaline Weak-Leaching Environment Only a portion of the mobile weathering products (silica, base cations) is lost by leaching in this situation. Aluminum and iron hydroxide are the least soluble weathering products, so these react with the soluble silica and base cations to produce 2 1 layer silicates, including dioctahedral smectites and illites. Chlorites can be formed in this situation as well. 

The laboratory synthesis at 2(fC of trioctahedral (Mg-bearing) layer silicate clays from solution has been found to be much easier than that of dioctahedral clays (e.g., kaolinite, montmorillonite). Provide a mechanism-based explanation for this, given that Al(OH)j is least soluble in the pH range of 5-9, while Mg(OH)2 is least soluble above pH 10. 

The 2 1 layer silicate minerals are sometimes defined on the basis of the number of octahedral positions occupied by cations. When two-thirds of the octahedral positions are occupied, such as in pyrophyllite (Al2Si40io(OH)2), the mineral is call dioctahedral, when all three positions are occupied, such as in talc (MgjSL OioCOH ), the mineral is called trioctahedral. This difference in composition of layer silicates can be fairly easily determined by x-ray diffraction, because each substitution slightly changes the dimensions of the unit cell. 

Drits V A (1975) The stmctural and crystallochemical features of layer silicates. In Ciystallochemistiy of Minerals, and Geological Problems. AG Kossovskaya (ed), Nauka, Novosibirsk, p 35-51 (in Russian) Drits VA, Besson G, Muller F (1995) An improved model for stmctural transformations of heat-treated aluminous dioctahedral 2 1 layer silicates. Clays Clay Minerals 43 718-731 Drits VA, Lindgreen H, Saly n AL (1997) Determination of the content and distribution of fixed anunonium in illite-smectite by X-ray diffraction Apphcationto North Sea ilhte-smectite. Am Mineral 82 79-87 Drits VA, Lindgreen H, Salyn AL, Ylagan R, McCarty DK (1998) Semiquantitative determination of trans-vacant and cis-vacant 2 1 layers in ilhtes and ilhte-smectites by thermal analysis and X-ray diffraction. Am Mineral 83 1188-1198... 

Muller F, Drits VA, Plangon A, Robert J-L (2000a) Structural transformations of 2 1 dioctahedral layer silicates dnring dehydroxylation-rehydroxylation reactions. Clays Clay Minerals 48 572-585 Muller F, Drits VA, Tsipursky SI, Plangon A (2000c) Dehydration of Fe, Mg-rich dioctahedral micas. (II) cation migration. Clay Mineral 35 505-514... 

Micas are layer silicates (phyllosilicates) whose structure is based either on a brucite-like trioctahedral sheet [Mg(OH)2 which in micas becomes Mg304(0H)2] or a gibbsite-like dioctahedral sheet [Al(OH)3 which in micas becomes Al204(0H)2]. This module is sandwiched between a pair of oppositely oriented tetrahedral sheets. The latter sheet consists of Si(Al)-tetrahedra which share three of their four oxygen apices to form a two-dimensional hexagonal net (Fig. 1). In micas, the association of these two types of sheet produces an M layer, which is often referred as the 2 1 or TOT layer. 

Daynyak LG, Drits VA, Heffits LM (1992) Computer simulation of cation distribution in dioctahedral 2 1 layer silicates using IR-data application to Mossbauer spectroscopy of a glauconite sample. Clays Clay Minerals 40 470-479... 

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